skin lightening

Over-the-counter skin-lightening products often contain 2% hydroquinone. 4% concentrations are available only by prescription from a physician

Unlike naturally derived skin lightening herbs, Hydroquinone is a strong inhibitor of melanin production, meaning that it lightens skin color.
Hydroquinone does not bleach the skin ; it only disrupts the synthesis of melanin production

Hydroquinone is considered the primary topical ingredient for inhibiting melanin production. Using it in combination with the other options listed can make a difference in skin discolorations.

Tyrosinase is rate-limiting enzyme for melanin synthesis, and defects in the enzymes activity lead to the albinism in human. It also seems likely that racial differences in human color may primarily be due to differences in the tyrosinase activity in the melanin cells from varying skin types. Melanin cells derived from black skin have up to ten times more tyrosinase activity and produce up to ten times more melanin than do melanin cells in white skin.

Melanin The Skin Pigment
Visible pigmentation in human results from the synthesis and distribution of melanin in the skin, hair bulbs and eyes. Melanin plays a crucial role in the absorption free radicals generated within the cytoplasm and in shielding the host from various types of ionizing radiation, including the UV light. Thus, melanin protect the skin against sun burn .
The essential enzyme in melanin biosynthetic pathway is tyrosinase, the only enzyme absolutely required for melanin production.

Tyrosinase, The Enzyme Behind The Dark Skin
Tyrosinase is multifunctional, glycosylated, copper-containing oxidase with a molecular weight of approximately 60 to 70 kDa. In mammals, it is exclusively found in melanin cells. It is therefore a good specific marker for these cells. Tyrosinase is synthesized in melanosomal ribosome's found on the rough endoplasmic reticulum. After synthesis, tyrosinase is glycosylated within Golgi, then delivered to melanosomes via coated vesicles .
Some concerns about hydroquinone's safety for skin lightening have been expressed, but the research indicates reactions are minor or a result of using extremely high concentrations and if an impure grade is used this can lead to skin damage.

"Overall, adverse events reported with the use of hydroquinone have been relatively few and minor in nature
To date there is no evidence of adverse systemic reactions following the use of hydroquinone for skin lightening. It has been around for over 30 years in skin-care products.
Skin lightening hydroquinone is undoubtedly the most active and safest skin depigmenting substance.

Hydroquinone can be an unstable ingredient in cosmetic formulations. When exposed to air or sunlight it can turn a strange shade of brown.
It is essential, when you are considering a hydroquinone product, to make sure it is packaged in a non-transparent container that doesn't let light in and minimizes air.Skin Lightening products packaged in jars are not recommended, because they become ineffective shortly after opening.